agent links: use matching emoji for every built-in icon

mara, #3762: stats already used a real emoji, the rest used plain
unicode arrow/hexagon glyphs — mixed style, and some of those glyphs
don't even render reliably (hexagon showed as a plain circle in one
repro). Picked a small matching set: compass/bar-chart/desktop/
hammer/gear. Operator-declared dashboardLinks extras keep their own
emoji unchanged.

frontend/packages/agent/src/components/MetaNav.tsx

@@ -8,7 +8,7 @@
 // stats/screen/forge/config + any `hyperhive.dashboardLinks` extras,
 // sourced from the backend's `agent_links()` (the single source of
 // truth — same list also feeds the dashboard card's icon strip) — plus
-// a `↑ dashboard` back-link, formerly the overflow menu's job (mara:
+// a `🧭 dashboard` back-link, formerly the overflow menu's job (mara:
 // "remove rebuild button, move link to dashboards into links menu" —
 // the overflow `⋯` trigger existed for exactly two items, dashboard-
 // link and rebuild; rebuild's gone outright, the dashboard's own
@@ -43,7 +43,7 @@ export interface MetaNavProps {
   links: AgentLink[];
   forgePublicUrl: string | null;
   /** Absolute base URL of the host dashboard (`resolveDashboardBase`)
-   * — the `↑ dashboard` item links to `${dashboardBase}dashboard.html`. */
+   * — the `🧭 dashboard` item links to `${dashboardBase}dashboard.html`. */
   dashboardBase: string;
 }
 
@@ -95,7 +95,7 @@ export function MetaNav({ links, forgePublicUrl, dashboardBase }: MetaNavProps)
             role="menuitem"
             onClick={() => setOpen(false)}
           >
-            ↑ dashboard
+            🧭 dashboard
           </a>
           {visible.map((lnk) => {
             const href = lnk.kind === 'forge' ? `${forgePublicUrl}${lnk.url}` : lnk.url;